Sensor assembly for magnetic flowmeter and method of manufacture

ABSTRACT

A sensor assembly for a magnetic flowmeter is provided that is cost-effective to manufacture, while incorporating an effective seal against leakage and enabling precise positioning of electrodes. The sensor assembly includes an electrode assembly and a plastic housing molded about the electrode assembly. The electrode assembly includes an electrode and a plastic base disposed about the lower end of electrode. The housing includes a wall in direct contact with the plastic base such that the tip of the electrode projects from the wall, facilitating a secure seal about the electrode. The sensor assembly further includes a magnetic assembly configured to provide a magnetic field extending beyond the wall of the housing.

BACKGROUND OF THE INVENTION

The present invention relates generally to magnetic flowmeters and, moreparticularly, to a magnetic flowmeter having electrodes in closeproximity to its electromagnet.

Magnetic flowmeters have long been used to measure the flow ofelectrically conductive fluids. Magnetic flowmeters are governed byFaraday's law of electromagnetic induction, which provides that aconductor passing through a magnetic field induces of voltage across theconductor. Typically, a magnetic flowmeter exposes a conductive fluid toa magnetic field whose plane typically is oriented transverse to thedirection of flow. As conductive fluid passes through the magneticfield, a voltage is induced. Such flowmeters typically include a pair ofelectrodes set apart from each other and in electrical contact with thefluid, to measure the induced voltage. Based on this measured voltage,the average velocity of the fluid can be determined.

Various configurations of flowmeters have been used, such as, bore-typeand insertion-type. In a typical bore-type configuration, magnetic coilsare mounted externally to a section of pipe and two electrodes extendinto the interior of the pipe at opposite sides. The electrodes form anaxis along the plane of the magnetic field, perpendicular to the fluidflow. Insertion-type flowmeters typically include a sensor assemblyhousing both a magnetic source and a pair of electrodes. In use, thesensor assembly is positioned with the electrodes in contact with thefluid flow.

For an accurate reading, it is beneficial to maintain a stable andstrong magnetic field within the measured region between the electrodes.Notably, the strength of a magnetic field increases relative to itsproximity to the magnetic source. Thus, it is beneficial to position theelectrodes and the magnetic source in close proximity to each other. Themagnetic source must also be sealed from the fluid, whereas theelectrodes are in contact with the fluid. Thus, the electrodes andadjacent structure must provide adequate sealing against leakage.

Various approaches have been implemented for sealing the magnetic sourcefrom the fluid. These approaches commonly incorporate fairly complexstructure, inhibiting proximity of the magnetic source to theelectrodes. Moreover, current approaches can be relatively expensive tomanufacture. Less expensive alternatives tend to sacrifice sensitivityand accuracy, which, in certain instances, can be attributed toimprecise positioning of the electrodes relative to one another orrelative to the magnetic source.

It should, therefore, be appreciated that there remains a need for amagnetic flowmeter that is cost effective to manufacture while providingprecise positioning of the electrodes relative to one another and inclose proximity to the magnetic source. It should also be appreciatedthat there remains a need for a magnetic flowmeter incorporating aneffective seal about the electrodes. The present invention fulfills thisneed and others.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the invention provides a sensor assemblyfor a magnetic flowmeter that is cost effective to manufacture, whileincorporating an effective seal against leakage and that enables precisepositioning of electrodes. The sensor assembly includes an electrodeassembly and a plastic housing molded about the electrode assembly. Theelectrode assembly includes an electrode and a plastic base disposedabout the lower end of electrode. The housing includes a wall in directcontact with the plastic base such that the tip of the electrodeprojects from the wall, facilitating a secure seal about the electrode.The sensor assembly further includes a magnetic assembly configured toprovide a magnetic field extending beyond the wall of the housing.

More specifically, by way of example and not limitation, the electrodehas an axial portion that includes the tip, and the lower end of theelectrode is oriented generally transverse to the axial portion. Boththe axial portion and the transverse portion can include notchesconfigured to interlock with the housing and the plastic base,respectively. Moreover, a portion of the plastic base can be fused withthe wall of the plastic housing, to aid in sealing.

In a detailed aspect of an exemplary embodiment, the base defines aprojection disposed about the electrode and projecting towards the tipof the electrode. The projection can be configured to encircle theelectrode. The projection can also be fused with the wall of thehousing, to aid in sealing.

In another exemplary embodiment in accordance with the invention, theelectrode assembly includes first and second electrodes spaced apartfrom each other and a plastic base disposed about lower ends of theelectrodes. The plastic base defines projections disposed about theelectrodes and projecting towards the tip of the electrode. The plastichousing is disposed about the electrode assembly such that the tips ofthe electrodes project from a distal end of the housing and that theplastic base contacts the end wall of the housing. The end wall of thehousing can include parallel ridges spaced apart from each otherdefining a valley such that the tips of the electrodes each project froma corresponding ridge of the end wall. The sensor assembly furtherincludes a magnetic assembly disposed within a cavity of the housing,proximate to the first and second electrodes. The magnetic assembly isconfigured to provide a magnetic field extending beyond the end wall ofthe housing.

The invention also provides a method of manufacturing a sensor assemblyfor a magnetic flowmeter. The method includes forming an electrodeassembly having a plastic base and molding a plastic housing about theelectrode assembly such that the plastic base is in contact with a wallof the housing.

In a detailed aspect of an exemplary method in accordance with theinvention, the method can include positioning a magnetic assembly withina cavity of the housing proximate to first and second electrodes. Themagnetic assembly preferably is configured to provide a magnetic fieldextending beyond a wall of the housing.

In other detailed aspects of exemplary methods, the electrode assemblyincludes first and second electrodes. The first and second electrodescan each have an axial portion and a lower end oriented generallytransverse to the axial portion. The plastic base can be molded aboutthe lower ends of the electrodes. In addition, a portion of the plasticbase can be become fused with the end wall of the plastic housing, as aresult of molding the housing about the electrode assembly.

In yet other detailed aspects of exemplary methods, the base is formedto define first and second projections disposed about a correspondingelectrode, each projection projecting towards the tip of the electrode.The projections can be configured to encircle the electrodes and tobecome fused with the wall of the housing, as a result of the moldingthe housing about the electrode assembly.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain advantages of the invention have beendescribed herein. Of course, it is to be understood that not necessarilyall such advantages may be achieved in accordance with any particularembodiment of the invention. Thus, for example, those skilled in the artwill recognize that the invention may be embodied or carried out in amanner that achieves or optimizes one advantage or group of advantagesas taught herein without necessarily achieving other advantages as maybe taught or suggested herein.

All of these embodiments are intended to be within the scope of theinvention herein disclosed. These and other embodiments of the presentinvention will become readily apparent to those skilled in the art fromthe following detailed description of the preferred embodiments havingreference to the attached figures, the invention not being limited toany particular preferred embodiment disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the following drawings in which:

FIG. 1 is a perspective view of an embodiment of a magnetic flowmetersensor assembly in accordance with the invention, depicting a twoelectrodes extending from a tip of a housing assembly and a groundingring disposed about the housing assembly.

FIG. 2 is a top plan view of the sensor assembly of FIG. 1.

FIG. 3 is a cross-sectional view of the sensor assembly of FIG. 1, takenalong line A—A of FIG. 2.

FIG. 4 is a cross-sectional view of the sensor assembly of FIG. 1, takenalong line B—B of FIG. 2.

FIG. 5 is a close-up view of area C of FIG. 3, depicting a tip portionthe sensor assembly of FIG. 1

FIG. 6 is a close-up view of area D of FIG. 4, depicting the tip portionthe sensor assembly of FIG. 1.

FIG. 7 is a top plan view of an electrode assembly of the sensorassembly of FIG. 1.

FIG. 8 is a first side elevational view of the electrode assembly of thesensor assembly of FIG. 1.

FIG. 9 is a second side elevational view of then electrode assembly ofthe sensor assembly of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawings, and particularly FIGS. 1, 2, and 7,there is shown an insertion-type sensor assembly 20 for a magneticflowmeter. The sensor assembly includes an electrode assembly 22 (FIG.7) having two electrodes 24 extending from a housing 26. The housing isover-molded about the electrode assembly such that the tips 28 of theelectrodes project from an end wall 30 of the housing, providing asecure seal to guard against leakage. The sensor assembly furtherincludes an electromagnet 32 disposed within a cavity of the housingproximate to the electrodes and a grounding ring 34 disposed about adistal end 36 of the housing.

In use, the electrodes 24 and the grounding ring 34 are in contact withthe fluid, while the electromagnet 32 generates a magnetic field beyondthe distal end of the sensor assembly. The magnetic field induces anelectrical current in the fluid perpendicular to the fluid flow. Theelectrodes are positioned to pick up the induced current. Since theelectromagnet and the electrodes are in close proximity (see, FIGS.3–4), the resulting measurement is particularly accurate.

The housing 26 includes two parallel ridges 38 that extend across theend wall 30 of the housing, defining a valley 40 across the diameter ofend wall. Each electrode projects from a mid point of a correspondingridge. A measuring line (A) across the valley can be defined by the tips28 of the electrodes. A reference arrow 40 (FIG. 1), is provided on aside wall 42 of the housing to ensure proper alignment of the sensorassembly relative to fluid flow, such that, the measuring line (A) isoriented transverse to the fluid flow. In this orientation, the valleyis aligned with the fluid flow, and the fluid flow is transverse to themagnetic field of the electromagnet 32.

With reference now to FIGS. 5 and 6, the electrodes 24 are T-shaped,including an axial portion 44 extending out the end wall 30 of thehousing and a transverse portion 46 disposed within a plastic base 48 ofthe electrode assembly. The electrodes are configured to facilitate asecure seal against fluid leakage into the cavity of the housing 26. Tothat end, each electrode has a portion of its length embedded in theplastic base, and the side wall of each electrode defines a plurality ofridges 50 encircling the electrode, providing a strong seal between theelectrode and the housing, given the length of the electrode. As bestseen in FIG. 6, the transverse portions of the electrodes define notches52 to facilitate a secure attachment to the plastic base.

The plastic base 48 includes an upper surface 54 that definesprojections, two collars 56, each collar encircling the axial portion 44of the corresponding electrode. The collars have a constant profile asthey encircle each axial portion, terminating in a pointed leading edge58. The collars are configured such that, during manufacturing, aleading edge 58 of each collar at least partially melts during themolding of the housing, further fusing the plastic base and the housingfor enhanced sealing about the electrodes. In other embodiments,projections of various other configurations can be provided about theelectrodes.

With reference now to FIGS. 7–9, the plastic base 48 has a generallyrectangular cross-section, defining a central opening 60. The plasticbase is formed about the electrodes such that the transverse portions 46are parallel to each other and have ends 62 extending out the sides ofthe base. The electrodes 24 are T-shaped, including the axial portion 44extending out the end wall of the housing and the transverse portion 46disposed within a plastic base of the electrode assembly. The axialportion includes a side wall 64 to aid in secure mating with the endwall of the housing. More particularly, the side wall defines notches 66that circumscribe the axial portion of the electrodes. The housing 26 ismolded about the electrode assembly such that the end wall conformswithin the notches of the axial portion, mechanically locking the endwall and the electrodes.

Four wires 68 are attached to the electrodes 24, providing conductivitybetween the electrodes and front-end electronics 70 (FIG. 2). In otherembodiments, different number of wires can be used. The front-endelectronics communicate with back-end electronics (not shown) to drivethe electromagnet 32 and process the signal from the electrodes. Twowires are attached to each electrode, which provides a certain measureof reliability and redundancy. In the exemplary embodiment, each wire isspot welded to a corresponding end 62 of the transverse portion of therespective electrode. In the exemplary embodiment, the electrodes areformed of stainless steel. In other embodiments, various other materialscan be used, as needed. For example, various Ni alloys have been foundto be beneficial, particularly for measuring flow of acids.

In an exemplary method of manufacture, the electrode assembly 22 and thehousing 26 are formed in separate molding stages. The electrodes arepositioned in a mold for forming the base about the transverse portionsof the electrodes, forming the electrode assembly. The electrodeassembly and the grounding ring 34 are then disposed in a moldconfigured to form the housing 26. The electrode assembly can be held inplace by a structure supporting the plastic base of the assembly. Themolding of the housing is conducted such that the leading edge of eachcollar 56 at least partially melts, fusing the base and the end wall,i.e., commingling material of the base and the end wall, whichfacilitates a secure seal about the electrodes. Moreover, the housing ismolded such that the end wall interlocks with the notches 66 of thetransverse portions the electrodes, further ensuring a secure seal. Inthis embodiment, the housing is formed of PVDF or polypropylene;however, other materials can be used. After molding, the electromagnet32 and the front-end electronics 70 are positioned in the cavity of thehousing. In the exemplary method, the molding of the plastic base andthe molding of housing are both performed a single shot, a singlematerial injection mold; however, multiple materials and shots can beused.

It should be appreciated from the foregoing that the present inventionprovides a sensor assembly for a magnetic flowmeter that is costeffective to manufacture, while incorporating an effective seal againstleakage and that enables precise positioning of electrodes. The sensorassembly includes an electrode assembly and a plastic housing moldedabout the electrode assembly. The electrode assembly includes anelectrode and a plastic base disposed about the lower end of electrode.The housing includes a wall in direct contact with the plastic base suchthat the tip of the electrode projects from the wall, facilitating asecure seal about the electrode. The sensor assembly further includes amagnetic assembly configured to provide a magnetic field extendingbeyond the wall of the housing.

Although the invention has been disclosed in detail with reference onlyto the exemplary embodiments, those skilled in the art will appreciatethat various other embodiments can be provided without departing fromthe scope of the invention. Accordingly, the invention is defined onlyby the claims set forth below.

1. A sensor assembly for a magnetic flowmeter, comprising: an electrodeassembly including an electrode and a plastic base, the electrode havinga tip and a lower end, the plastic base disposed about the lower end ofthe electrode; a plastic housing molded about the electrode assembly toseal against leakage about the electrode, the housing including a wallin direct contact with the plastic base, the tip of the electrodeprojecting from the wall, wherein a portion of the plastic base is fusedwith the wall of the plastic housing; and a magnetic assembly configuredto provide a magnetic field extending beyond the wall of the housing. 2.A sensor assembly as defined in claim 1, wherein an axial portion of theelectrode includes the tip, and the lower end of the electrode isoriented generally transverse to the axial portion.
 3. A sensor assemblyas defined in claim 1, wherein a side wall of the electrode definesnotches mechanically locked with the wall of the housing.
 4. A sensorassembly as defined in claim 1, further comprising a grounding elementconfigured to be in contact with fluid, when in use.
 5. A sensorassembly as defined in claim 1, wherein the base defines a projectiondisposed about the electrode and projecting towards the tip of theelectrode, the projection in contact with the wall of the housing.
 6. Asensor assembly as defined in claim 5, wherein the projection encirclesthe electrode.
 7. A sensor assembly as defined in claim 5, wherein theprojection is fused with the wall of the housing.
 8. A sensor assemblyfor a magnetic flowmeter, comprising; an electrode assembly having,first and second electrodes spaced apart from each other, each electrodehaving a tip and a lower end, and a plastic base disposed about thelower ends of the first and second electrodes, the base defining firstand second protrusions, each protrusion disposed about a correspondingelectrode; a plastic housing molded about the electrode assembly anddefining a cavity, the housing including an end wall in contact with theplastic base, the tips of the electrodes projecting from the end wall;and a magnetic assembly disposed within the cavity of the housingproximate to the first and second electrodes, the magnetic assemblyconfigured to provide a magnetic field extending beyond the end wall ofthe housing.
 9. A sensor assembly as defined in claim 8, wherein thefirst electrode and the second electrode each have an axial portionincluding the tip, the lower end of each electrode is oriented generallytransverse to the axial portion.
 10. A sensor assembly as defined inclaim 8, wherein a portion of the plastic base is fused with the endwall of the plastic housing.
 11. A sensor assembly as defined in claim8, wherein the end wall of the housing includes parallel ridges spacedapart from each other defining a valley.
 12. A sensor assembly asdefined in claim 8, wherein the tips of the electrodes each project froma corresponding ridge of the end wall.
 13. A sensor assembly as definedin claim 8, wherein each projection encircles the correspondingelectrode.
 14. A sensor assembly as defined in claim 13, wherein theprojections are fused with the end wall of the housing.